How is a triangle used in construction?

A triangle is used in construction to make sure something is square or to square something up. A speed square is a small triangular shaped tool used to mark rafter angles. One method to check the square of a room or one wall that is perpendicular to another is to measure 3 feet along one wall and make a pencil mark them measure 4 feet along the other wall and make a pencil mark. Then measure diaganally from pencil mark to pencil mark. If the number is 5 feet the two walls are square to each other.